1. Field of the Invention
The present invention relates to a unidirectional prepreg, carbon fiber reinforced resin composite materials, and processes for producing the sames.
2. Description of the Related Art
Carbon fiber reinforced resin composite materials (hereinafter referred to as "CFRP") are light weight and have a high specific strength and high specific modulus and are utilized in various fields including sports and leisure products, aerospace structures, general industrial machines, etc.
CFRPs are produced from carbon fibers using a filament winding method, prepreg lamination method, pultrusion method or the like, but the prepreg method is predominately used to obtain high performance CFRPs.
Prepregs are an intermediate sheet material obtained by impregnating carbon fibers with an uncured resin such as an uncured epoxy resin, and a number of such prepregs are laminated followed by heating and pressing the laminate to form same into a specific shape. This is the prepreg lamination method.
Unidirectional prepregs with carbon fibers unidirectionally arranged are predominately used to obtain a high performance CFRP, because cloth prepregs, in which carbon fibers are woven and braid prepregs in which carbon fibers are braided, do not perform adequately due to the undulation of fibers and local stress concentration caused thereby.
At present PAN-based carbon fibers produced from polyacrylonitrile (PAN) as a starting material are currently used as the carbon fibers in CFRPs. Nevertheless, the PAN-based carbon fibers are expensive because the polyacrylonitrile fibers as a starting material are expensive and the yield of carbonization is low; accordingly, the application of PAN-based carbon fibers is limited to aircraft and airospace products, and sports and leisure products, etc.
On the other hand, pitch-based carbon fibers produced from carbonaceous pitch as a starting material have the advantage of being low in price because the starting material is inexpensive and the yield of carbonization is high. Particularly, mesophase pitch-based carbon fibers produced from mesophase pitches containing more than 40%, preferably 60% of mesophase have attracted attention as they are inexpensive and provide a high performance carbon fiber.
It is known that mesophase pitch-based carbon fibers may have a high modulus because a highly oriented and highly graphitized carbon fiber can be easily produced by utilizing the orienting and graphitizing properties of a mesophase pitch as the starting material. For example, a mesophase pitch based carbon fiber with carbon layer planes three dimensionally developed, is highly graphitized and has a high modulus, and a process for producing the same, are disclosed in JP-A-49-19127.
Nevertheless, it was reported that CFRPs using mesophase pitch-based carbon fibers (hereinafter referred to as "pitch-based carbon fiber(s)") have a relatively low compressive strength compared to its tensile strength and that as the modulus of a pitch-based carbon fibers is increased, the compressive strength of the fibers is reduced. Accordingly, improvement of the compressive strength of the pitch-based carbon fibers is essential for applying the pitch-based carbon fibers to primary structural materials. The above are particularly applicable when pitch-based carbon fibers having a modulus in tension of 400 GPa or more are used in a CFRP.
As reported in Journal of Materials Science, 27, 3782(1992), it was recently revealed that the compressive stress-strain curve of a unidirectional CFRP using high modulus carbon fibers has non-linearity in that the slope of the curve gradually decreases as the strain increases. Generally, for pitch-based carbon fibers having a modulus in tensile of 400 MPa or more, the slope of the compressive-stress-strain curve decreases to about 60% of the initial slope when failure occurs due to compression. Accordingly, it is considered that in a compressive strength measurement using planer plates, the test piece fails in the mode of shear buckling or the like so that the compressive strength and compression strain at failure are measured to be small.
In the case of the above-mentioned pitch-based carbon fibers, the tensile strength of the fibers is not less than 2000 MPa but the compressive strength of the unidirectional CFRPs is not more than 800 MPa.
The hybrid method, in which different fibers are combined, is known to improve the physical properties of a weaker carbon fiber. For example, JP-B-02-42098 published in 1991 discloses an improvement of the bending strength of the weaker carbon fiber reinforced plastics by combining stronger grade carbon fibers for reinforcement, but the description of the fiber dispersion of the weaker and stronger fibers and the specific methods of production are not disclosed. It also mentions that reinforcement fibers should advantageously have a diameter as large as possible.
JP-A-02-292337 published in 1992 discloses an improvement of the bending strength of carbon fibers by uniformly dispersing additional different fivers having high compressive strength carbon fibers. It also affirms that uniform dispersion of the carbon fibers and the addition of fibers other than carbon fibers are essential for improving the compressive strength.
JP-A-02-292337 also confirms that the so-called in-layer hydrid in which regions of carbon fibers and regions of different fibers (non-carbon fibers) adjacent to each other are arranged in a layer and the so-called interlayer hybrid in which carbon fiber layers and different fiber layers are laminated, but in these hybrids, the fibers are broken at low strain values and improvement of the compressive strength of the hybrids is thus insufficient.
Accordingly, uniform dispersion of large diameter fibers is essential for improving the compressive strength of carbon fibers by the hybrid method. An apparatus for attaining such uniform dispersion and productivity thereof rendered the hybrid fibers expensive and the necessary large diameter fibers made the thin prepregs, necessary for the production of fishing rods or golf shafts, difficult.
The object of the present invention is to provide CFRPs capable of enduring repeated loads and having an improved compressive strength and a unidirectional, if necessary thin prepreg for providing such a CFRP.